Process for the extraction of metal values with bisimidazole metal extractants

ABSTRACT

Metal values are extracted from aqueous solutions containing halide or pseudohalide anion using 1,1&#39;-substituted 2,2&#39;-biimidazole or bibenzimidazole compounds. Useful for the solvent extraction of zinc and copper from chloride leach solutions.

This invention relates to a process for the extraction of metal valuesfrom aqueous solutions of metal salts, and in particular to a processfor the extraction of metal values from aqueous solutions in thepresence of halide anions.

The use of solvent extraction techniques for the hydrometallurgicalrecovery of metal values from metal ores has been practised commerciallyfor a number of years. For example copper may be recovered from oxideores or from ore tailings by treating the crushed ore with sulphuricacid to give an aqueous solution of copper sulphate which issubsequently contacted with a solution in a water-immiscible organicsolvent of a metal extractant whereby the copper values are selectivelyextracted into the organic phase.

The application of solvent extraction techniques to aqueous solutionscontaining halide anions however has presented numerous technicalproblems. For example copper bearing sulphur-containing ores such aschalcopyrite may be leached using ferric chloride or cupric chloridesolutions, but the solvent extraction of the resultant leach solutionspresents formidable difficulties. The recovery of zinc by solventextraction from halide-containing solutions such as those derived fromsulphur-containing ores by chloride leaching has also been proposed (Seefor example, G. M. Ritcey, B. H. Lucas and K. T. Price, Hydrometallurgy,1982, 8, page 197). However, known extractants for zinc (for exampleorganophosphorous compounds such as tributyl phosphate) generally show apoor efficiency of metal recovery and a poor selectivity for zinc overthe iron present in such leach solutions.

The present invention provides a process for the extraction of metalvalues from aqueous solutions containing halide or pseudohalide ions bythe use of metal extractants whose several properties meet the stringentrequirements imposed on the extractant by the system.

According to the present invention there is provided a process forextracting metal values from aqueous solutions of metal salts containinghalide or pseudo anions which comprises contacting the aqueous solutionwith a solution in a water-immiscible organic solvent of a biimidazoleor bibenzimidazole of formula: ##STR1## wherein X and Y, which may thethe same or different, taken together contain a total of from 12 to 52saturated carbon atoms and are each selected from the group R--, RCO--,--CH₂ CO.OR, --CH(CO.OR)₂ and --CO.OR wherein R is a hydrocarbyl group;or wherein X and Y taken together contain a total of from 12 to 52saturated carbon atoms and taken together are selected from the groups##STR2## wherein R is a hydrocarbyl group; and wherein A, B, C and D,which may be the same or different are each hydrogen or a substituent Zwherein Z is lower alkyl, halogen, nitro, cyano or--CO.OR' wherein R' islower alkyl; or A and B taken together with the two carbon atoms joiningthem form an optionally substituted cyclohexene ring and/or C and Dtaken together with the two carbon atoms joining them form an optionallysubstituted cyclohexene ring; or A and B taken together with the twocarbon atoms joining them form an optionally substituted benzene ringand/or C and D taken together with the two carbon atoms joining themform an optionally substituted benzene ring.

The term `lower alkyl` as used herein means an alkyl group containingfrom 1 to 6 carbon atoms and preferably from 1 to 4 carbon atoms.

The group R is preferably an alkyl, optionally substituted aryl,optionally substituted alkylaryl, optionally substituted arylalkyl,optionally substituted cycloalkyl or optionally substituted alkoxyalkyl.It is to be noted that X and Y may be the same or different, and hencedifferent groups R may be present in X and Y. Especially preferredgroups R are branched alkyl groups containing from 8 to 24 carbon atoms,provided that the groups X and Y together contain from 12 to 52saturated carbon atoms.

The groups X and Y are preferably the same. Preferred groups X and Y arethe group--CO.OR. Thus it is especially preferred that the groups X andY are the same and are both--CO.OR wherein R is a branched chain alkylgroup containing from 8 to 24 carbon atoms. Improved solubility indesired solvents may often be achieved when the group R is a mixture ofalkyl groups and especially an isomeric mixture of alkyl groupscontaining the same number of carbon atoms.

When A and B taken together with the two carbon atoms joining them areoptionally substituted cyclohexene rings, C and D may each be hydrogenor one of the substituents Z defined above. Alternatively, both A and Btaken together with the two carbon atoms joining them and C and D takentogether with the two carbon atoms joining them may be optionallysubstituted cyclohexene rings. As examples of optional substituentswhich may be present in the cyclohexene ring(s) there may be mentionedthose substituents Z as defined above.

When A and B taken together with the two carbon atoms joining them areoptionally substituted benzene rings, C and D may each be hydrogen orone the substituents Z defined above. Alternatively, both A and B takentogether with the two carbon atoms joining them and C and D takantogether with the two carbon atoms joining them may be optionallysubstituted benzene rings. As examples of optional substituents whichmay be present in the benzene ring(s) there may be mentioned thosesubstituents Z as defined above. An especially preferred class ofcompounds of formula I above are 2,2'-bibenzimidazoles of formula:##STR3## wherein S and S' are separately hydrogen or any of thesubstituents as defined above for Z. S and S' are preferably hydrogen,lower alkyl (for example methyl) or halogen. It is preferred that the 4and 4' positions in the bibenzimidazole are free of substitution. Thusin especially preferred compounds of formula (II), S and S' areseparately hydrogen or methyl (in a position other than 4,4', or 7,7')and X and Y represent the group CO.OR in which R is a hydrocarbyl groupas defined. In a preferred group of such compounds, S and S' arehydrogen and X and Y are the same and are each the group--CO.OR whereinR is a branched primary alkyl group or an isomeric mixture of branchedprimary alkyl groups. By the term `branched primary alkyl group` as thatterm is used herein is meant a branched alkyl group bearing two hydrogenatoms on the carbon atom linked to the oxygen atom in the group--CO.OR.We have found that such compounds generally have an especially highaffinity for zinc combined with good selectivity against acid transfer.

2,2'-Biimidazole compounds for use in the present invention may beprepared by reaction of (optionally substituted) 2,2'-biimidazole withappropriate alkyl or acyl halides or alkylchloroformate to introduce thegroups X and Y.

2,2'-Bibenzimidazole compounds for use in the present invention may beprepared by reaction of an appropriately substituted o-phenylenediaminewith an appropriate derivative of oxalic acid. Trichloroacetonitrile ormethyl trichloroacetamidate are particularly convenient as the oxalicacid derivative (Holan, Ennis and Hinde, J. Chem. Soc. (London), (C)1967 page 20).

2-(2-Imidazolyl)-benzimidazole compounds for use in the presentinvention (that is compounds in which either A and B or C and D but notboth form part of a benzene ring) may be prepared as described byYutilov and Kovaleva (Russian Pat. No. 541846 -Chemical Abstracts No.171448g Vol. 86, 1977) or by the procedure of Ennis, Holan and Samuel(J. Chem. Soc. (C), 1967 pages 33 to 39).

Compounds for use in the invention in which A and B taken together withthe carbon atoms joining them and/or B and C taken together with thecarbon atoms joining them represent cyclohexene rings may be prepared bycatalytic hydrogenation of the corresponding benzenoid compounds or fromthe corresponding biimidazolines (Duranti and Balsamine, Synthesis 1974page 815) or as described by Bernarducci et al, Inorganic ChemistryVolume 22, 1983, pages 3911 to 3920).

Highly branched primary alkyl groups R may be usefully derived fromcommercially available mixtures of branched aliphatic alcoholsmanufactured by the `Oxo` process or from branched alcohols prepared bythe Guerbet and Aldol condensations. Such Guerbet alcohols are primaryalcohols characterised by branching at the position beta to the hydroxylgroup and have the general formula: ##STR4## wherein R₁ and R₂ are bothalkyl groups and R₁ contains two fewer carbon atoms than R₂. R₁ and R₂may be straight chain or branched chain alkyl groups and may be isomericmixtures of alkyl groups. A mixture of highly branched alcohols may beobtained by Guerbet or Aldol condensations of mixtures of alcohols andaldehydes respectively. By way of Example, good solubility in preferredorganic solvents is conferred on biimidazole or bibenzimidazolecompounds of formula (I) above wherein the group --R as defined in X andY is derived from commercial isooctadecanol prepared by the aldoldimerisation of commercial nonanol and believed to consist essentiallyof a mixture of geometrical isomers of the compound (IV): ##STR5##

Such alcohols may be used directly to form the correspondinggroup--CO.OR. Alternatively the alcohols may be oxidised to carboxylicacids and thence converted to the corresponding acid chloride tointroduce the group--CO.R, for example by reaction with2,2'-bimididazoles as described above. It will be appreciated of coursethat once the alcohol is oxidised to the corresponding carboxylic acid,the derived alkyl group R will no longer be a primary alkyl group asdefined above.

The process of the present invention may be applied to the extractionfrom aqueous solutions containing halide or pseudohalide ion of anymetal capable of forming a stable halide or pseudohalide containingcomplex with the biimidazole or bibenzimidazole compound in thewater-immiscible organic solvent. Examples of such metals include zinc,copper, cobalt, and cadmium. The process of the present invention isespecially suitable for the solvent extraction of zinc and copper fromaqueous solution obtained by the halide or pseudohalide leaching ofsulphur containing ores. In general, such ores contain both copper andzinc in relative proportions which vary from ore to ore. It isconvenient to recover both copper and zinc in successive processingstages from the leach solutions. For example the copper may be recoveredfrom the leach solution by solvent extraction and the raffinate fromthis process may be treated in a separate solvent extraction process forthe recovery of zinc. The process of the present invention may be usedto recover either copper or zinc, although the same biimidazole orbibenzimidazole compound will not necessarily be used as the extractantin both cases. Preferably the process of the present invention is usedin the zinc extraction stage only and a different solvent extractant(for example a solvent extractant disclosed in European PatentApplication No. 0 057 797) used in the copper extraction stage.

It will be appreciated that the process of the present invention may beincorporated into a wide variety of different methods for the overallrecovery of metals from their ores or from other metal-bearing sources.Details of these methods will vary depending on the metal concerned andthe nature and composition of the leach solution. By way of example, anintegrated process which is especially suitable for leach solutionscontaining high levels of cupric ion is described in European PatentApplication No. 0 507 797.

The biimidazole or bibenzimidazole compounds for use in the presentinvention are especially useful for the recovery of zinc which hashitherto proved to be very difficult to recover effectively by solventextraction. Thus the compounds of the present invention have in generala high affinity for zinc, which in general is combined with an excellentselectivity for zinc over acid and iron which are inevitably present inthe leach solution especially for example when ferric chloride is usedas leachant. However, even compounds which have a high affinity for zincalso retain a moderately high affinity for copper, and such reagents arenot truly selective for zinc in the presence in high levels of copper.This is not a significant disadvantage in practice even when it isdesired to recover zinc from a solution containing both copper and zinc,since the recovery of zinc generally takes place after the bulk ofcopper has been removed, for example in a first solvent extractionstage. If the reagent has a high affinity for both zinc and copper, itis perfectly feasible to remove residual copper remaining in theraffinate, for example by cementation using a metal such as zinc oriron, and to treat the resulting copper-free aqueous solution by solventextraction to recover the zinc. However, the compounds of use in theprocess of the present invention generally show a surprisingly highrelative selectivity for zinc over copper such that it may be possibleto treat the solution remaining after copper solvent extraction withoutthe need for an additional cementation step to remove residual copper.

The zinc solvent extraction circuit may be similar in design to thatproposed in European Patent Application No. 0 057 797 for the recoveryof copper from halide containing solutions by solvent extraction. Thusfor example in a circuit for the recovery of copper and zinc from theaqueous leach solution derived from the leaching of a sulphur-containingore with for example ferric chloride, the aqueous low-copper raffinatefrom the copper solvent extraction stage will contain zinc, iron andhalide ion (for example 35 gpl zinc, 70gpl iron, 3.7 M in chloride ionand containing 5gpl hydrochloric acid). This feed to the zinc circuitmay be contacted with a solution of the extractant of the presentinvention in a water immiscible organic solvent into which the zinc isextracted. The loaded organic phase solution is contacted with anaqueous strip solution containing a reduced level of zinc and halide ionsuch that at least a proportion of the zinc transfers into the aqueousstrip phase. The stripped organic phase is returned to extract morezinc, and the loaded aqueous strip solution is passed to a zinc recoverystage, typically an electrowinning stage. The electrowinning stage mayproduce metallic zinc and chlorine gas (as described for example in`Zinc Electrowinning from Chloride Electrolyte` by D. J. MacKinnon andJ. M. Brannen; Mining Engineering April 1982 page 409) which may be usedto regenerate the ferric chloride leachant (now reduced to ferrous ion).Alternatively an internal regeneration of the leachant may take place ina split cell without the generation of free chlorine gas. The zinc andchloride ion depleted aqueous stream from the electrowinning stage isreturned to the strip stage to act as the aqueous strip solution,thereby completing the zinc extraction circuit.

The extraction process of the present invention may be represented by anequation such as the following:

    L.sub.org +M.sup.++.sub.aq +2Cl.sup.-.sub.aq ⃡(LMCl.sub.2).sub.org

where M is a divalent metal ion such as zinc.

This equation is a grossly oversimplified representation of a verycomplex process and is not to be taken as in any way limiting the scopeof the present invention, but it serves to illustrate the formation of aneutral organic phase complex of the divalent metal and the extractant(L) which is believed to predominate in the process of the presentinvention. The equation illustrates the reversible nature of theextraction, whereby the complex of the metal and the extractant in theorganic phase can be stripped on contact with the aqueous solution fromthe electrowinning stage which is depleted in the metal and in thehalide ion.

A further property which is of importance for an extractant in theprocess of the present invention is the absence of significantprotonation by the acidic leach liquor. Such protonation may berepresented by an equation such as:

    L.sub.org +H.sup.+.sub.aq +Cl.sup.-.sub.aq ⃡(LH.sup.+ Cl.sup.-).sub.org

where L is the extractant. Such protonation of the ligand carrieshydrochloric acid into the organic phase and builds up an excessivechloride ion concentration on the strip side. We have found that thisproblem is particularly acute for the extraction of zinc which isthought to promote the acid transfer. Preferred reagents of the presentinvention combine a high affinity for zinc with a low acid transfer intothe organic phase.

As illustrated by the Examples, the extractants of the present inventionprovide a range of properties so that the optimum extractant may beselected for a given leach solution.

Examples of suitable water-immiscible organic solvents are aliphatic,aromatic and alicyclic hydrocarbons, chlorinated hydrocarbons such asperchloroethylene, trichloroethane and trichloroethylene. Mixtures ofsolvents may be used. Especially preferred in conventionalhydrometallurgical practice are mixed hydrocarbon solvents such as highboiling, high flash point petroleum fractions (for example kerosene)with varying aromatic content. In general, hydrocarbon solvents having ahigh aromatic content, for example AROMASOL H which consists essentiallyof a mixture of trimethylbenzenes and commercially available fromImperial Chemical Industries PLC (AROMASOL is a trade mark) or SOLVESSO150 commercially available from Esso (SOLVESSO is a trade mark), providea higher solubility for the extractant and its metal complex, whilstkerosene having a relatively low aromatic content, for example ESCAID100 which is a petroleum distillate comprising 20% aromatics, 56.6%paraffins and 23.4% naphthenes commercially available from ESSO (ESCAIDis a trade mark) may in certain cases improve the hydrometallurgicalperformance of the extractant. Factors influencing the solubility of theextractant and its metal complex are complicated, but in generalextractants having highly branched substituents and/or an isomericmixture of substituents have comparatively high solubility. Theconcentration of the extractant in the water-immiscible organic solventmay be chosen to suit the particular leach solution to be treated.Typical values of extractant concentration in the organic phase arebetween about 0.1 to 2 Molar, and an especially convenient range is from0.2 to 1.0 Molar in the organic solvent.

Certain biimidazole or bibenzimidazole for use in the present inventionare novel compounds and the present invention includes such novelcompounds.

The invention is illustrated by the following Examples in which allparts and percentages are by weight unless otherwise stated.

EXAMPLE 1

This Example illustrates the preparation and use of1,1'-bis(tridecyloxycarbonyl)-2,2'-bimidiazole.

The tridecanol used in this Example was the commercially availablemixture of isomeric branched-chain C₁₃ primary aliphatic alcoholsmanufactured by the oxo process. Tridecyl chloroformate was prepared byadding tridecanol (200g) dropwise during 75 minutes to liquid phosgene(160g) which was refluxing below a condenser cooled with a mixture ofsolid carbon dioxide and acetone. After addition was complete themixture was stirred for 30 minutes. Excess phosgene was then allowed toevaporate into a scrubbing device charged with 20% aqueous sodiumhydroxide. The last traces of phosgene were removed by bubbling nitrogenthrough the liquid while it was heated to 80° .

Tridecyl chloroformate (26.2g) was added dropwise to a stirredsuspension of 2,2'-biimidazole (6.7g) in a mixture of dry methylenechloride (35cm³) and dry pyridine (25cm³) at such a rate that thetemperature did not exceed 35° . After stirring for 16 hours at roomtemperature the reaction mixture was filtered and methylene chloride wasdistilled from the filtrate under reduced pressure. The residue wasextracted with petroleum ether (200cm³, bp 60-80° ) and the petroleumextract was extracted with two equal volumes of hydrochloric acid (2M)and then with water until acid free. The petroleum solution was driedwith magnesium sulphate and the solvent was distilled under reducedpressure yielding an oil (25.3g). The nmr spectrum of this oil, andtitration of an aliquot with perchloric acid in acetic acid and aceticanhydride, confirmed that it comprised 65% by weight of1,1'-bis(tridecyloxycarbonyl)-2,2'-biimidazole and 35% of tridecanol. Itwas used as an extractant without further purification.

The ability of this compound to extract copper and zinc from solutionscontaining chloride ion was investigated by the following generalmethod:

An aqueous solution was prepared which was 0.1M in cupric chloride(6.35gpl copper), and 0.1M in hydrochloric acid and which contained250gpl of calcium chloride dihydrate. This solution was then agitatedfor 1.5 minutes with an equal volume of a solution which was a 0.2Msolution of the extractant in SOLVESSO 150. The layers were allowed toseparate and settle, and were separately analysed for copper content.The transfer of copper from the aqueous to the organic phase wascalculated as the percentage of the ligand taken up as the coppercomplex (assuming the complex LCuCl₂). The transfer of hydrochloric acidfrom the aqueous solution into the organic solution was calculated asthe percentage of ligand that was protonated, (assuming thestoichiometry LH⁺ Cl⁻). The test was repeated using different molaritiesof hydrochloric acid and different concentrations of calcium chloride,and the results are presented in Table 1.

A similar test was used to determine the ability of the extractant toextract zinc from aqueous solution. An aqueous solution was preparedwhich was 0.6M in zinc chloride (39.24gpl zinc) and 0.1 M inhydrochloric acid and which contained 176.5gpl of calcium chloridedihydrate (the calcium chloride concentration was adjusted to takeaccount of the higher zinc chloride concentration). This solution wasthen agitated for 1.5 minutes with an equal volume of a solution whichwas a 0.2M solution of the extractant in SOLVESSO 150. The layers wereallowed to separate and settle, and were separately analysed for zinccontent. The transfer of zinc from the aqueous to the organic phase wascalculated as the percentage of the ligand taken up as the zinc complex(assuming the complex LZnCl₂). The transfer of hydrochloric acid fromthe aqueous solution into the organic solution was calculated as thepercentage of ligand that was protonated. The test was repeated usingdifferent molarities of hydrochloric acid and different concentrationsof calcium chloride, and the results are presented in Table 2.

The results show that the product of this Example is a very powerfulextractant for both zinc and copper. The results also show anexceptionally low degree of acid transfer from acidic solutioncontaining zinc chloride.

EXAMPLE 2

This Example illustrates the preparation and use of1,1'-bis(isooctadecyloxycarbonyl)-2,2-bibenzimidazole.

Following the general procedure of Example 1, isooctadecyl chloroformatewas prepared by reacting isooctadecanol with phosgene. Isooctadecylchloroformate (99.8g) was then reacted with a suspension of2,2-bibenzimidazole (35.lg) in a mixture of dry methylene chloride(100cm³) and dry pyridine (50cm³) yielding after work up1,1'-bis(isooctadecyloxycarbonyl)-2,2-bibenzimidazole (124g, 86% purity)as a viscous oil.

The compound was evaluated as an extractant for copper and zinc usingthe procedure of Example 1 and the results are presented in Tables 1 and2 respectively. The results show that the compound has good affinity forzinc, with a very low level of acid transfer, and relatively lowaffinity for copper.

EXAMPLE 3

Following the procedures of Examples 1 and 2, 2-hexyldecyl chloroformatewas prepared from 2-hexyldecanol and reacted with 2,2-bibenzimidazole togive 1,1'-bis(2-heptyldecyloxycarbonyl)-2,2'-bibenzimidazole as an oilin 82% purity.

The compound was evaluated as an extractant for copper and zinc usingthe procedure of Example 1 and the results are presented in Tables 1 and2 respectively. The results show that this compound is very similar inproperties to the product of Example 2. Compared with the product ofExample 2 it showed faster phase disengagement during the tests.

EXAMPLE 4

Following the procedure of Example 2, tridecylchloroformate was reactedwith 2,2'-bibenzimidazole to give 1,1'-bis(tridecyloxycarbonyl)-2,2'-bibenzimidazole in 86% purity.

The compound was evaluated as an extractant for copper and zinc usingthe procedure of Example 1 and the results are presented in Tables 1 and2 respectively. The results show that this compound is similar inproperties to the product of Example 2, but that it has the advantage ofbeing a slightly stronger ligand for zinc. Like the product of Example3, it was less viscous and showed good phase disengagement properties.

EXAMPLE 5

This Example illustrates the preparation and use of1,1'-bis(isodecyloxycarbonyl)-2,2'-bibenzimidazole

The isodecanol used in this Example was the commercially availablemixture of isomeric branched chain C₁₀ primary aliphatic alcoholsmanufactured by the oxo process. Following the procedures of Examples 1and 2, isodecyl chloroformate was prepared from isodecanol and reactedwith 2,2'-bibanzimidazole to give 1,1'-bis(isodecyloxycarbonyl)-2,2'-bibenzimidazole in 65% purity.

The compound was evaluated as an extractant for copper and zinc usingthe procedure of Example 1 and the results are presented in Tables 1 and2 respectively. The results show that this compound is similar to theproduct of Example 4.

EXAMPLE 6

This Example illustrates the preparation and use of dimethyl substituted1,1'-bis(tridecyloxylcarbonyl)-2,2'-bibenzimidazole.

A mixture of isomeric dimethyl-2,2'-bibenzimidazoles was prepared asfollows. 40.7g of a mixture comprising commercially available1,2-diamino-4-methylbenzene (66% w/w) and 1,2-diamino-3-methylbenzene(33% w/w) was dissolved in methanol. The solution was stirred and methyl2,2,2-trichloroacetimidate (20.6cm³) was added dropwise during 30minutes at 17° . Stirring was continued for 16 hours and the mixture wasthen boiled under reflux for 30 minutes, and then allowed to cool. Theprecipitate was collected, washed with methanol and dried yielding apale fawn solid (18.2g, mp 286-291° ).

This material was reacted with tridecyl chloroformate using theprocedure of Example 2, yielding an oil in 81% purity having the generalformula given below which represents an isomeric mixture of dimethylsubstituted compounds: ##STR6##

This mixture was evaluated as an extractant for copper and zinc usingthe procedure of Example 1 and the results are presented in Tables 1 and2 respectively. The results show that whilst this mixture is a ratherweaker liqand for zinc than the products of Examples 1 to 5, it showsexcellent freedom from acid transfer

                  TABLE 1                                                         ______________________________________                                                                    % Uptake from                                             HCl    CaCl.sub.2.2H.sub.2 O                                                                       0.1 M CuCl.sub.2                                 Example   Molarity (g/l)        Copper                                                                              HCl                                     ______________________________________                                        1         0.1      250          49    0                                       2         0.1      250          0     0                                                 0.1      700          5     0                                                 1.0      700          4     2                                       3         0.1      250          0     0                                                 1.0      700          6     3                                       4         0.1      250          0     0                                                 1.0      700          7     1                                       5         0.1      250            0.5 0                                                 1.0      700            6.5   1.5                                   6         0.1      250          0     0                                                 1.0      700          5     1                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                    % Uptake from                                             HCl    CaCl.sub.2.2H.sub.2 O                                                                      0.6 M ZnCl.sub.2                                  Example   Molarity (g/l)        Zinc  HCl                                     ______________________________________                                        1         0.01     176.5        97    0                                                 0.01     626.5        98    0                                                 0.10     626.5          97.5                                                                              0                                       2         0.01     176.5        23    0                                                 0.01     626.5        33    0                                                 0.10     626.5        35    0                                       3         0.01     176.5          27.5                                                                              0                                                 0.01     626.5        33    0                                                 0.10     626.5        37    0                                       4         0.01     176.5          24.5                                                                              0                                                 0.01     626.5          33.5                                                                              0                                                 0.10     626.5        37    0                                       5         0.01     176.5        23    0                                                 0.01     626.5        31    0                                                 0.10     626.5          32.5                                                                              0                                       6         0.01     626.5        19    0                                                 0.10     626.5        20    0                                       ______________________________________                                    

EXAMPLE 7

This example demonstrates the selectivity of a typical compound of theinvention in extracting zinc from an aqueous feed solution containingother metal impurities. The feed solution corresponded to the liquorobtained by (i) leaching a complex sulphide ore with ferric chloridesolution and (ii) removing copper originally present in the liquor, forexample by solvent extraction. The Example also demonstrates that thezinc can be stripped from the loaded organic solution by contact with adilute aqueous solution of zinc chloride so as to give a moreconcentrated and purified aqueous solution of zinc chloride suitable asan advance electrolyte for the electrowinning of zinc. The dilutesolution of zinc chloride used in stripping is representative of thespent electrolyte recovered after electrowinning has taken place.

A first feed solution was prepared to contain 5gpl HCl and the amountsof each metal listed in the first row of Table 3. A second more stronglyacidic feed solution was made up having the same metal contents, but20gpl HCl. The first feed solution was contacted by stirring for 15minutes with an equal volume of a 0.5 molar solution of the product ofExample 2 in ESCAID 100. The organic layer was separated and thenstripped by contacting with an equal volume of an aqueous solutioncontaining 15gpl zinc (as ZnCl₂) and 5gpl HCl. The aqueous solution wasthen separated and analysed for metals by atomic absorption spectroscopywith the results listed in the second row of Table 3. This procedure wasrepeated with the second, more acidic, feed solution, yielding anadvance electrolyte having the composition listed in the third row ofTable 3.

                                      TABLE 3                                     __________________________________________________________________________           Zn.sup.II                                                                        Fe.sup.II                                                                        Pb.sup.II                                                                        Sb.sup.III                                                                       Sn.sup.II                                                                        Cd.sup.II                                                                        As.sup.III                                                                       Ag.sup.I                                                                         Co.sup.II                                                                        Ni.sup.II                                   __________________________________________________________________________    Feed   35 75 1.6                                                                              140                                                                              40 110                                                                              180                                                                              30  30                                                                              70                                          Composition                                                                          gpl                                                                              gpl                                                                              gpl                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                         Advance                                                                              23.5                                                                              7  7  5  2 0.6                                                                              <2  2 <1 <0.5                                        Electrolyte                                                                          gpl                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                         (from                                                                         5 gpl HCl                                                                     feed)                                                                         Advance                                                                              22.5                                                                              8 2   13                                                                               4 0.7                                                                               4 1  <1 <0.5                                        Electrolyte                                                                          gpl                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                                                              ppm                                         (from                                                                         20 gpl HCl                                                                    feed)                                                                         __________________________________________________________________________

We claim:
 1. A process for extracting metal values from aqueoussolutions of metal salts containing halide or pseudo halide anions whichcomprises contacting the aqueous solution with a solution in awater-immiscible organic solvent of a biimidazole or bibenzimidazoleextractant of formula: ##STR7## wherein X and Y, which may be the sameor different, taken together contain a total of from 12 to 52 saturatedcarbon atoms and are each selected from the group R--, RCO--, --CH₂CO.OR, --CH(CO.OR)₂ and --CO.OR wherein R is a hydrocarbyl group; orwherein X and Y taken together contain a total of from 12 to 52saturated carbon atoms and taken together are selected from the groups##STR8## wherein R is a hydrocarbyl group; and wherein A, B, C which maybe the same or different are each hydrogen or a substituent Z wherein Zis lower alkyl, halogen, nitro, cyano or--CO.OR' wherein R' is loweralkyl; or A and B taken together with the two carbon atoms joining themform an optionally substituted cyclohexene ring and/or C and D takentogether with the two carbon atoms joining them form an optionallysubstituted cyclohexene ring; or A and B taken together with the twocarbon atoms joining them form an optionally susbstituted benzene ringand/or C and D taken together with the two carbon atoms joining themform an optionally substituted benzene ring.
 2. A process according toclaim 1 wherein the group R is an alkyl, optionally substituted aryl,optionally substituted alkylaryl, optionally substituted arylalkyl,optionally substituted cycloalkyl or optionally substituted alkoxyalkyl.3. A process according to claim 1 wherein the groups R are the same andare branched alkyl groups containing from 8 to 4 carbon atoms, providedthat the groups X and Y together contain from 12 to 52 carbon atoms. 4.A process according to claim 1 wherein X and Y are the group COOR and Ris a branched chain alkyl group containing from 8 to 24 carbon atoms. 5.A process according to claim 1 wherein the groups R are a mixture ofisomeric alkyl groups.
 6. A process according to claim 1 wherein A and Btaken together with the two carbon atoms joining them are optionallysubstituted cyclohexene rings, C and D are each hydrogen or one of thesubstituents Z.
 7. A process according to claim 1 wherein A and B takentogether with the two carbon atoms joining them and C and D takentogether with the two carbon atoms joining them are optionallysubstituted cyclohexene rings.
 8. A process according to claim 1 whereinA and B taken together with the two carbon atoms joining them areoptionally substituted benzene rings, C and D are each hydrogen or oneof the substituents Z.
 9. A process according to claim 1 wherein A and Btaken together with the two carbon atoms joining them and C and D takentogether with the two carbon atoms joining them are optionallysubstituted benzene rings.
 10. A process according to claim 1 whereinthe extractant is of the formula ##STR9## wherein S and S' areseparately hydrogen or a substituent Z.
 11. A process according to claim10 wherein the X and Y represent the group COOR and S and S' areseparately hydrogen or methyl.
 12. A process according to claim 1wherein the C02R group is derived from primary alcohols characterised bybranching at the position beta to the hydroxyl group and have thegeneral formula: ##STR10## wherein R1 and R2 are both alkyl groups andR1 contains two fewer carbon atoms than R2.
 13. A process according toclaim 1 wherein the extractant is1,1'-bis(tridecyloxycarbonyl)-2,2'-biimidazole.
 14. A process accordingto claim 1 wherein the extractant is1,1'-bis(isooctadecyloxycarbonyl)-2,2-bibenzimidazole.
 15. A processaccording to to claim 1 wherein the extractant is1,1'-bis(2-heptyldecyloxycarbonyl) -2,2'-bibenzimidazole.
 16. A processaccording to claim 1 wherein the extractant is1,1'-bis(tridecyloxycarbonyl)-2,2'-bibenzimidazole.
 17. A processaccording to claim 1 wherein the extractant is1,1'-bis(isodecyloxycarbonyl)-2,2'-bibenzimidazole.
 18. A processaccording to claim 1 wherein the extractant is a dimethyl substituted1,1'-bis(tridecyloxylcarbonyl)-2,2'bibenzimidazole.